1. Field of the Invention
This invention relates to improvements in a continuously variable transmission incorporating therein a toroidal type continuously variable transmission utilized, for example, as a transmission for an automobile.
2. Description of the Related Art
U.S. Pat. No. 5,888,160 discloses a continuously variable transmission which includes an input shaft rotatively driven by a drive source, an output shaft for taking out power based on the rotation of this input shaft, a toroidal type continuously variable transmission having input and output discs and a power roller which are interposed between the input shaft and the output shaft, and a planetary gear mechanism.
This continuously variable transmission has two modes for a low speed side and a high speed side during forward movement, wherein the mode for the low speed side utilizes a power transmission system routed through only the toroidal type continuously variable transmission and the mode for the high speed side utilizes a power transmission system routed through the toroidal type continuously variable transmission and a power transmission system not routed through the toroidal type continuously variable transmission. The arrangement provided is such that these two power transmission systems are inputted to any two gears of a sun gear, a ring gear, and planetary gears of a planetary gear mechanism, the remaining one gear is coupled with the output shaft, and an output is obtained as the differential component of the two gears. Since the power transmitted through the toroidal type continuously variable transmission becomes small in the mode for the high speed side, this continuously variable transmission offers the advantages of becoming highly efficient and having long life.
With the continuously variable transmission disclosed in U.S. Pat. No. 5,888,160, the torque inputted to the toroidal type continuously variable transmission during a mode change for effecting changeover between the low speed mode and the high speed mode changes greatly from the plus side to the minus side (or vice versa). For example, the input torque during a mode change in which the mode for the low speed side is changed over to the mode for the high speed side changes from +350 Nm to −280 Nm.
Also, in the geared neutral system wherein two power transmission systems are utilized in the mode for the low speed side and a mode in which only the toroidal type continuously variable transmission is routed through is utilized in the mode for the high speed side, and wherein in the mode for the low speed side the differential component of the planetary gears is set to 0 rotation to make a starting clutch unnecessary, the inversion of positive and negative sides of the torque similarly occurs when the two modes are changed over. Incidentally, in the geared neutral system, the mode change is effected when the toroidal type continuously variable transmission is on the low speed side.
In addition to these systems, there is a continuously variable transmission for producing two or modes by combining the toroidal type continuously variable transmission and planetary gears, but the positive and negative sides of the torque are inverted in many layouts at the time of the mode change. For example, a two-mode continuously variable transmission such as the one disclosed in U.S. Pat. No. 6,251,039 is present.
However, the toroidal type continuously variable transmission has a characteristic that when the torque is changed, the speed changes as shown in FIG. 7. FIG. 7 shows the results of measurement at a time when the number of revolution was fixed to 2000 revolutions or thereabouts, the oil temperature and the like were controlled at a temperature close to that during actual vehicle running, and no speed change commands were issued, but only the torque was varied. If the torque is varied in this manner, the toroidal type continuously variable transmission undergoes a speed change despite the fact that no speed change has been issued.
The following reasons are conceivable as the causes of this speed change.    (1) When a load is applied to the toroidal type continuously variable transmission, a traction force occurs in the vertical direction (the direction of the axis of inclination of the trunnion supporting the power roller), the force of the piston of a mechanism for moving the trunnion acts in the opposite direction as a reaction force thereof. Of course, gaps are present in a radial needle roller bearing supporting the power roller and a radial needle roller bearing supporting a pivot. For this reason, when a load is applied, the inner ring of the power roller moves in the vertical direction by the aggregate sum portion of these gaps. As this power roller moves in the vertical direction, side slippage occurs, with the result that the power roller undergoes a speed change.    (2) In addition, the transmission ratio changes with respect to the load of the torque due to the effect of the deflection of a trunnion shaft caused by the resilient deformation of the trunnion. In a traction drive, it is necessary to cause a pressing force to act on the traction contact point, and that force is supported by the trunnion.
In addition, the trunnion is supported by two yokes, and forces occurring between two portions of the trunnion in the back-and-forth direction and the left-and-right direction are canceled. Accordingly, the trunnion assumes a state in which a load is applied to a beam supported at two points, so that the trunnion naturally undergoes resilient deformation. Consequently, although a trunnion shaft itself is not resiliently deformed since it is not subjected to forces, but the trunnion shaft is affected by the resilient deformation of the trunnion, with the result that the trunnion shaft becomes inclined. As a result, the point of contact between a precess cam of a mechanism for moving the trunnion and a valve link moves, so that the spool moves in the axial direction. In consequence, the valve is cut, and the differential pressure occurs, resulting in the occurrence of the speed change.
Due to the combination of these factors, as shown in FIG. 7, on application of the torque, the toroidal type continuously variable transmission undergoes a speed change despite the fact that no speed change command has been issued.
If the variation of speed change is of such a nature that it occurs stably, it is possible to suppress the occurrence of a shock in speed change by issuing a speed change command. However, the situation must be avoided in which control becomes unstable at the time of the mode change, i.e., when the torque has varied. Incidentally, at the time of the mode changeover, the driver does not intend to make the mode change, and the mode change takes place while the driver is unaware. Accordingly, if the time duration of connection and disconnection of the clutch is long in the mode change, the state becomes such that no power is being generated, so that the driver feels a sense of uncomfortableness due to such as the revving up of the engine.
From the above, it can be understood that the mode change must be effected stably, and at the same time that the mode change needs to be completed by momentarily shifting the clutch in a short time.